The present invention relates to electrical contact manufacturing methods and apparatus. More specifically, the invention relates to an electrical contact manufacturing method in which an electrical contact in the form of a spot welded to a predetermined position on a base metal plate is raised together with the base metal plate to increase the service life of the contact.
Electrical contacts in electromagnetic switches are formed as spot-like contacts made of a noble metal such as silver. Such contacts are welded to predetermined points on base metal plates made of copper or brass. In order to weld a contact to the base metal plate, a staking method, a brazing method and a resistance welding method have previously been employed. In the resistance welding method, current is made to flow between the base metal plate and the contact material which are disposed between two electrodes and the base metal plate and the contact material are welded together under pressure. The resistance welding method is extensively employed. In welding a relatively small contact material to a base metal plate using the resistance welding method, the contact material which is, for instance, a silver wire, is held with one of the electrodes while the end of the contact material is welded to the surface of the base metal plate on the other electrode while being depressed against the plate. Thereafter, the contact material is cut to a desired length to provide a contact. The contact thus formed is shaped under pressure with a shaping punch.
In the conventional contact manufacturing method, the shaping punch placed on the contact welded to the base metal plate is strongly pressed towards the base metal plate as a result of which a portion of the contact cuts into the base metal plate. The volume of the portion is, in general, about 10 to 15% of the total volume of the contact.
When a switch having such contacts is repeatedly operated, the contacts are gradually worn beginning with the top surface and the circumferential surface such as shown by phantom lines 11, 12 and 13 as indicated in FIG. 8. When the contact is worn to the line 13, it is necessary to replace it with a new one. However, such replacement is not economical because the worn contact still has a volume of 20% of its initial volume including the aforementioned portion which is cut into the base metal plate. Thus, the replacement or disposal of the worn contact is a waste of the expensive contact material.
Accordingly, an object of the invention is to provide an electrical contact manufacturing method in which the entrance of the contact into the base metal plate is prevented during the resistance welding process thereby increasing the service life of the contact.
Another object of the invention is to provide an electrical contact manufacturing apparatus for practicing the method.